Nucleation and evolution of Si1−xGex islands on Si(001)

In this study, we report a systematic investigation of the metastable morphologies of Si_1−xGe_x layers obtained by the interplay of kinetics and thermodynamics during growth on Si(001). We show that three main growth regimes can be distinguished as a function of the misfit and of the deposited thickness. They correspond to three equilibrium steady state morphologies that consist of (105)-facetted hut islands, huts and domes in co-existence, and a bimodal size distribution of domes, respectively. The shape transitions between these states are attributed to different levels of relaxation.     

Molecular beam epitaxy synthesis of Si1−yCy and Si1−x−yGexCy alloys and Ge islands using an electron cyclotron resonance argon/methane plasma

We report the growth of Si_1−yC_y and Si_1−x−yGe_xC_y alloys on Si(001) by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy using an argon/methane gas mixture. Various Si/Si_1−yC_y and Si/Si_1−x−yGe_xC_y multilayers have been grown and characterized principally by X-ray diffraction and Raman spectroscopy. The influence of growth parameters and electron cyclotron resonance plasma source operating conditions on the C substitutional incorporation was studied. Under optimum growth conditions the structures show good structural properties and sharp interfaces with carbon being essentially substitutionally incorporated up to concentrations of 1%. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation. Si_1−x−yGe_xC_y layers grown with this technique exhibit the strain compensation and enhanced thermal stability expected for these ternary alloys. Carbon pre-deposition of Si through surface exposure to the argon/methane plasma is shown to act as an antisurfactant on the growth of Ge islands by suppressing the formation of a Ge wetting layer on the surface.     

Doping and electrical characteristics of in-situ heavily B-doped Si1−x−yGexCy films epitaxially grown using ultraclean LPCVD

Doping and electrical characteristics of in-situ heavily B-doped Si_1−x−yGe_xC_y (0.22<x<0.6, 0<y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550°C in a SiH₄–GeH₄–CH₃SiH₃–B₂H₆–H₂ gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH₄ partial pressure, and only at high GeH₄ partial pressure did it decrease with increasing B₂H₆ as well as CH₃SiH₃ partial pressures. With the B₂H₆ addition, the Ge and C fractions scarcely changed and the B concentration (C_B) increased proportionally. The C fraction increased proportionally with increasing CH₃SiH₃ partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si_1−x−yGe_xC_y with y=0.0054 or below, the carrier concentration was nearly equal to C_B up to approximately 2×10²⁰ cm⁻³ and was saturated at approximately 5×10²⁰ cm⁻³, regardless of the Ge fraction. The B-doped Si_1−x−yGe_xC_y with high Ge and C fractions contained some electrically inactive B even at the lower C_B region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially.     

Magnetism in Si1 − xMnx diluted magnetic semiconductor thin films

We have studied the electrical and magnetic properties of p-type semiconductor thin films of Si_1 − xMn_x/Si (x = 0.036 and 0.05) grown by molecular beam epitaxy. Experimental results reveal that the resistivity of the samples decreases gradually with increasing measurement temperature, which can be described well by Mott's variable-range-hopping model. All the samples exhibit the ferromagnetic ordering above room temperature. Among these samples, Si_0.95Mn_0.05 has a higher hole density and magnetization. This indicates an enhancement of hole-mediated ferromagnetic exchange interactions when the Mn-doping concentration is increased.     

Study of optical constants in CdxSe1−x

Thin films of Cd_xSe_1−x (0<x<1) have been deposited by the vacuum evaporation technique onto ultra clean substrates maintained at 300°C. The optical properties (specially transmission spectra) of these films were studied. The wavelength dependence of refractive index and extinction coefficient of the films along with the variation of these constants with composition was studied.     

Thermal stability and crystallization of amorphous Si1−xBx, Si1−xPx and Si1−xSbx alloy thin films

The thermal stability and crystallization of the amorphous Si_1−xP_x, Si_1−xB_x and Si_1−xSb_x alloy systems have been studied by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). It was found that the amorphous Si-P and Si-B alloys have a high thermal stability and crystallize at temperatures as high as 1150°C, whereas amorphous Si-Sb alloys are unstable and crystallize already at 350°C. The thermal stability is explained by strong Si-P and Si-B bonds, whilst the instability is explained by the weakening of Si-Si bonds by the presence of Sb atoms (< 17 at.% Sb) and by weak Si-Sb and Sb-Sb bonds (> 17 at.% Sb). The thermal stability and instability of the amorphous alloys were correctly predicted from calculated Gibbs free energy diagrams (GFED). The calculated GFED were also used together with the observed crystallization temperatures to successfully predict the first crystalline phase to form in the Si-P and Si-B systems. The first crystalline phases that formed were Si (< 40 at.% B and < 30 at.% P), a newly reported phosphide, Si₁₂P₅ (30 at.% P), and SiP (> 30 at.% P).     

Photoelectrochemical investigations of n-CdSe1−xTex thin film electrodes

An electrode/electrolyte interface has been formed between an n-type CdSe_1−xTe_x (0≤x≤1) alloyed/mixed type semiconductor and a sulphide/polysulphide redox electrolyte. It has been investigated through the current–voltage, capacitance–voltage and spectrally selective properties. The dependence of the dark current through the junction and the junction capacitance on the voltage across the junction have been examined and analysed. It appeared that the current transport mechanism across the junction is strongly influenced by the recombination mechanism at the interface and series resistance effects. Upon illumination of the interface with a light of 20 mW cm⁻², an open circuit voltage of the order of 0.35 V and a short circuit current of 212 μA cm⁻² have been developed (for x=0.2), yielding an efficiency of energy conversion equal to 0.2% and a form factor of 45%. The action spectra in the 450–1000 nm wavelength range showed presence of the interface states at the electrode/electrolyte interface. The magnitudes of the barrier heights at the interfaces were also determined. It has been seen that a significant improvement in the electrochemical performance of a cell is noticed for the electrode composition with x=0.2.     

Phase relations, lattice thermal expansion in Ce1−xEuxO2−x/2 and Ce1−xSmxO2−x/2 systems and stabilization of cubic RE2O3 (RE: Eu, Sm)

The phase relations in CeO₂–Eu₂O₃ and CeO₂–Sm₂O₃ systems have been established under slow-cooled conditions from 1400 °C. The two-phase relations differ as the CeO₂–Eu₂O₃ system showed only two monophasic phase fields, namely F-type cubic and C-type cubic, whereas CeO₂–Sm₂O₃ system showed three phase fields namely F-type cubic, C-type cubic and a biphasic field comprising of C-type cubic and monoclinic phase. An interesting observation of this investigation is the stabilization of C-type rare-earth oxide after Ce⁴⁺ substitution, which is attributed to decrease in average cationic size on Ce⁴⁺ substitution at RE³⁺ site. The lattice thermal expansion behavior of F-type solid solution and C-type solid solution in CeO₂–Eu₂O₃ system was investigated by high-temperature XRD.     

Strong magnetoelastic effect in Pr1−xCaxMnO3

Manganese oxides with distorted perovskite structure have attracted much attention during the last decade due to their colossal magnetoresistance (CMR), and the strong correlations among the various degrees of freedom involved. In particular, Pr_1−xCa_xMnO₃ compounds present in a wide Ca-doping range a charge ordering phenomenon, consisting of real space ordering of Mn³⁺ and Mn⁴⁺ in alternate lattice sites below a certain temperature T_CO. This ordering brings about a lattice distortion and a large hardening of the sound velocity below T_CO. Tomioka et al. have observed that an applied magnetic field can melt this charge ordered state and induce a transition from an insulating to a metallic state. In order to study the effects of this charge order melting, ultrasonic longitudinal sound velocity measurements were performed on polycrystalline Pr_1−xCa_xMnO₃ (x=0.35 and 0.5) as a function of magnetic field and temperature. Interesting anomalies were found related to the melting of the charge ordered phase into a metal-like state even at low temperatures.     

Combinatorial synthesis and rapid characterization of Mo1−xSnx (0x1) thin films

Thin films of Mo_1−xSn_x, continuously and linearly mapped for 0<x<1, have been prepared by d.c. magnetron sputter deposition under various growth conditions. X-ray diffraction results indicate that as x in high-pressure deposited Mo_1−xSn_x increases from 0 to approximately 0.45, the bcc lattice expands and no new phases are formed. At low deposition pressures, Mo₃Sn, a β-tungsten structured phase, is formed along with the bcc Mo–Sn solid solution for 0.1<x<0.3. The variation of the lattice parameter for this intermetallic phase also indicates that solid solutions, possibly of the form Mo_3+ySn, are being formed. These materials are of special interest as anode candidates in lithium-ion batteries.     

Bond-lengths of the atomic nearest-neighbor and next-nearest-neighbor in A1−xBxC1−yDy III–V solid solutions

On the basis of the FDUC model and the hypothesis of the constant covalent radii, the expressions of the atomic nearest-neighbor and the next-nearest-neighbor bond-lengths were derived for A_1−xB_xC_1−yD_y III–V quaternary solid solutions. This set of bond-length expressions predicts the averaged bond-lengths and bond angles at any concentration (x, y) for the III–V pseudobinary and quaternary solid solutions, which are only dependent on the lattice parameters and the concentrations of the pure end compounds. When x=0, 1 or y=0, 1, A_1−xB_xC_1−yD_y III–V quaternary solid solutions degenerate into the relative pseudobinary solid solutions, in which the nearest-neighbor and the next-nearest-neighbor bond-lengths agree well with the experimental results. Further discussion and comparison with other theoretical models are also given in this paper.     

Photoconductors based on ZnxCd1−xS and CdSe1−ySy thin films, fabricated with multilayer structure

Photoconductors based on evaporated Zn_xCd_1−xS and CdSe_1−yS_y thin films, with a novel multilayer structure, were fabricated and characterized through spectral response measurements. The device structure is constituted by several layers of different energy gaps, sequentially deposited side by side and interconnected in series or in parallel. The influence of the preparation conditions, number of layers and electrode configuration, on the performance of the photoconductors was determined. Photoresistors with a detection range between 400 and 750 nm were developed, using four layers with energy gaps varying between 1.8 eV and 2.8 eV and interconnected in parallel.     

Crystallization kinetics of SbxSe100−x thin films

The crystallization kinetics in Sb_xSe_100−x films with 39≤x≤58is studied by monitoring the optical transmission of the films during both isothermal and constant rate heatings. The structure of the films upon crystallization and at certain intermediate stages is studied by electron microscopy techniques. The results are analyzed in the frame of the Johnson-Mehl-Avrami theory in order to determine the kinetic parameters (Avrami exponent, activation energy and frequency factor) in addition to the crystallization temperature. The results show that film crystallization is always preceeded by a relaxation process which modifies substantially the optical properties of the amorphous material. Amorphous films with compositions close to the stoichiometric compound (Sb₂Se₃) are found to show the highest activation energy for crystallization.     

Electronic structure and transport properties of Co(Si1−xMx) (M = Al, P): First-principles study

In this study, by using the full-potential linear augmented plane wave (FLAPW) method based on the density functional theory (DFT), the lattice parameter of CoSi was calculated theoretically and the calculations of the electronic structures of CoSi and CoSi_1−xM_x (M = Al, P and x = 0.03125, 0.125) were performed. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semi-metal and the density of states (DOS) is very small at the Fermi level. M-doping can tune the Fermi level and the hole pockets and the electron ones, which is very valuable to modulate the transport properties. Based on the calculated electronic structures and our experimental results on CoSi [C.C. Li, W.L. Ren, L.T. Zhang, K. Ito, J.S. Wu, J. Appl. Phys. 98 (2005) 063706], the intrinsic relations between electronic structures and transport properties of CoSi and CoSi_1−xAl_x are discussed in detail. The transport properties along main crystallographic directions of binary CoSi and CoSi_1−xAl_x are experimentally examined. The experimental results show that the electrical resistivity of CoSi-based compounds is anisotropic, while the Seebeck coefficient is almost isotropic. The calculated band structures of CoSi_1−xAl_x can theoretically interpret the anisotropy of the electrical transport properties.     

Phase formation process of IrxSi1−x thin films Structure and electrical properties

Experimental data on the phase formation process of amorphous Ir_xSi_1−x thin films with 0.30 ≤ x ≤ 0.41 are presented and discussed in relation to electric transport properties. The structure formation process at temperatures from 300 K up to 1223 K was investigated by means of X-ray diffraction. Distinct phases were observed in the final stage in dependence on the initial composition: Ir₃Si₄, Ir₃Si₅, and IrSi₃. An unknown metastable phase was found in films with a silicon concentration of 61 at.% to 64 at.% after annealing above the crystallization temperature T = 970 K. The crystal structure of this phase was determined by the combined use of X-ray diffraction and electron diffraction. It was found to be monoclinic, basic-face centred with lattice constants a = 1.027 nm, b = 0.796 nm, c = 0.609 nm, and γ = 113.7°. Additionally, microstructure and morphology of the films were investigated by transmission electron microscopy (TEM). The annealing process was studied by means of mechanical stress investigations as well as by electrical resistivity and thermopower measurements. Correlations between the structure, the phase formation and the electrical transport behaviour are discussed on the basis of conduction mechanism.     

Random stacking in MoS2−x sputtered thin films

The structure of sputter-deposited MoS_2−x films is investigated by X-ray and electron diffraction. The diffraction spectra show important differences with respect to those of known molybdenite phases. These differences are interpreted in terms of the random-layer model usually applied to turbostratic carbon. Peak profile calculations are performed following an extension of Wilson's model, taking into account the structure factor. They show that the crystal structure of these films consists in a random stacking of S---Mo---S layers, without evidence of 2H or 3R polytypism.     

High-temperature oxygen non-stoichiometry, conductivity and structure in strontium-rich nickelates La2−xSrxNiO4−δ (x = 1 and 1.4)

Oxygen non-stoichiometry, electrical conductivity and thermal expansion of La_2−xSr_xNiO_4−δ phases with high levels of strontium-substitution (1 ≤ x ≤ 1.4) have been investigated in air and oxygen atmosphere in the temperature range 20–1050 °C. These phases retain the K₂NiF₄-type structure of La₂NiO₄ (tetragonal, space group I4/mmm). The oxygen vacancy fraction was determined independently from thermogravimetric and neutron diffraction experiments, and is found to increase considerably on heating. The electrical resistivity, thermal expansion and cell parameters with temperature show peculiar variations with temperature, and differ notably from La₂NiO_4±δ in this respect. These variations are tentatively correlated with the evolution of nickel oxidation state, which crosses from a Ni³⁺/Ni⁴⁺ to a Ni²⁺/Ni³⁺ equilibrium on heating.     

Epitaxial grown K1−xRbxTiOPO4 films with extremely flat surfaces for waveguiding

We report on epitaxial {1 0 0} K_1−xRb_xTiOPO₄ waveguide films for the visible spectral range grown on KTiOPO₄ substrates by liquid phase epitaxy. Using the m-line technique a refractive index increase of Δn_x≈0.007 and Δn_z≈0.004 for TM and TE polarisation has been determined for a K_0.78Rb_0.22TiOPO₄ film. Optical transmission and nearfield distribution are comparable to conventional ion-exchanged waveguides. Typical attenuation of about 1 dB/cm for both TM and TE polarisation was obtained at λ=532 and 1064 nm. Energy-dispersive X-ray spectrometry reveals solid-solution films with graded rubidium composition profiles. X-ray rocking curve analyses confirm the epitaxial growth process and indicate perfect and relaxed K_1−xRb_xTiOPO₄ films. Atomic force microscopy investigations reveal regular step structures with step heights Δh<1.3 nm resulting in rms-roughness values of ≈0.4 nm.     

Sol–gel preparation of boron-containing cordierite Mg2(Al4−xBx)Si5O18 and its crystallization

Five cordierite-based powders were investigated regarding their thermal and crystallization behaviors. The powders were obtained from amorphous gels having nominal compositions of 2Mg : xAl : (4 − x)B : 5Si where x = 4 down to 0. Thermal gravimetry analysis of the dry gels showed some absorbed water and decomposition of organic ligands in addition to network condensation. Gradual substitution of B for Al in the dried gel powders showed a new band in their infrared spectra corresponding to triangular BO₃, whereas the bands corresponding to Al vanished. This also showed a noticeable effect on the crystallization trends, type and stability of cordierite. Cordierite crystallized in samples of B/Al ratio up to 1 while protoenstatite predominated in samples of higher B/Al ratios. In addition, some silica minerals, with little amorphous phase, were formed. Incorporation of boron and increase in temperature enhanced the transformation of γ cordierite to its form.     

Fabrication of a-Si1−xCx:H thin films for solar cells by the Cat-CVD method using a carbon catalyzer

We propose a new approach to fabrication of hydrogenated amorphous silicon carbide (a-Si_1−xC_x:H) thin films for solar cells by the catalytic chemical vapor deposition (Cat-CVD) method using a carbon catalyzer, which is more stable than tungsten or tantalum. It was found that by using the carbon catalyzer, undoped and boron-doped a-Si_1−xC_x:H films were easily obtained from a SiH₄, CH₄ and B₂H₆ mixture without any change in the catalyzer surface, even after deposition for longer than 30 h.